A wide variety of microchips use caps to protect their interior components. For example, micro-electromechanical systems (“MEMS devices”) often have a cap to protect their fragile microstructure. Many MEMS devices typically have a glass seal to bond the silicon die caps to the underlying MEMS chip. Such a seal, which can be hermetic, may have widths on the order of about 150 to 400 microns. Undesirably, this seal footprint increases die size. As a consequence, fewer dies/microchips can be formed from individual wafers, thus increasing per-unit fabrication costs.
The art has responded to this problem by using metal to bond the cap dies/wafers to the underlying device dies/wafers. Specifically, metal can provide bonds and hermeticity comparable to seal glass, but with smaller widths. Accordingly, metal bonds/seals effectively increase the number of dies that can be formed on a single wafer. To those ends, conventional processes often heat the metal to its melting point and apply pressure to produce a strong bond.